Population Growth

Over the past 50 years, the developing countries (as conventionally defined by the United Nations) added 3.2 billion people to the world's population, which has now reached 6.0 billion. Over the next 50 years, if current moderate projections hold, this same group of countries will add another 3.1 billion people to the world's population. Substantial population growth does not seem to have gone away.

Fifty years is a long period for a projection. However, shorter projections show the same trends. From 1975 to 2000, developing-country populations will have grown by 1.9 billion people (reaching 4.9 billion). From 2000 to 2025, they will continue to grow by an additional 1.8 billion people. Figure 1 shows population growth by decade in the industrial and developing countries. The peak decade for population growth in developing countries is the 1990s. Even if, as projected, growth falls off a little, it will still be well over 700 million per decade over the next 20 years, and then will decline only gradually. Industrial countries, on the other hand, will contribute little to world population growth and could as a whole experience negative growth toward the middle of the next century.

Projections naturally involve some uncertainty but are not pure speculation. After all, of the population in the year 2025, more than half have already been born, and of the population a decade from now, at least 80 percent are already among us. Historical experience with patterns of childbearing can be applied to estimate the remaining proportions.¹

¹Such projections almost always assume that current attempts to reduce the rate of population growth will continue and will not be weakened. If, instead, fertility were to cease to decline, the developing-country population would grow by 2.4 billion, not 1.8 billion, in 2000 to 2025, and by 5.8 billion, not 3.1 billion, in 2000 to 2050.

Figure 1 - Population Increments by Decade in Industrial and Developing Countries (millions)

SOURCE: United Nations (1996) and projections using the World Bank (1997a) model.

The projections show continued substantial growth despite the fact that fertility has moderated in developing countries. In the 1950s total fertility was estimated at 6.0 children per woman. By the first half of the 1990s, it had fallen almost by half, to 3.3 children per woman. Annual population growth, about 2.1 percent a year in the 1950s, is below 1.8 percent and falling.

Despite these fertility declines and even assuming they will continue, the annual increments to developing-country populations will still be large. While fertility has fallen overall, very high fertility persists in many countries: Total fertility in Uganda, for instance, has been estimated recently at 6.9 children per woman, and it is only one of about two dozen countries with fertility at 6.0 or higher. About another two dozen countries have fertility between 5.0 and 5.9 (World Bank, 1997a). In addition, annual increments are large because developing-country populations are much bigger now and are predominantly young, with a median age of about 23. Young populations include many potential childbearers, whose children in turn will continue to swell the population. As a result, the number of babies born in developing countries should keep increasing for another two decades. The population growth rate will moderate during this period only because, as the population ages, the number of deaths will also go up.

If each couple were to have no more children than those necessary to replace themselves, developing-country populations would still continue to grow because of their young age structures. This is illustrated in Figure 2 with population pyramids, which show the distribution of population by age and sex. Each cohort (each group of people born in contiguous years) moves upward in the pyramid as it ages, and the upper levels of the pyramid therefore expand over time, as the contrast between 1995 and 2025 shows. Even if each cohort only reproduces itself (so that the lowest rung of the pyramid resembles the preceding ones), population will still grow, especially at older ages. This phenomenon, known as population momentum, will account for almost three-quarters of the population growth in developing countries over the next 25 years and for practically all the growth in East Asia. Momentum is largely absent, by contrast, in the pyramid for industrial countries (the last pyramid), which has a narrow base.²

²Momentum can also be compared to the long-term interest on the national debt, continuing almost indefinitely into the future. The debt exists because of a previous excess of government expenses over revenue. Momentum exists because of a previous societal "excess" of births over deaths. The quicker a balanced budget is reached, the smaller the long-term debt. Similarly, the quicker a balance of fertility and mortality is reached, the smaller the long-term population momentum. If a balanced budget cannot be reached this year, reducing the deficit will at least slow the growth in the national debt. Likewise, reducing fertility will at least slow the increase in population momentum. Even if the government achieves a balanced budget, and keeps it balanced indefinitely, interest charges will continue on the accumulated debt. Even if fertility stays at replacement level indefinitely, population will continue to grow because of the accumulated momentum. Finally, just as only applying a government budget surplus to the principal on the national debt will reduce future interest payments, only fertility below replacement will reduce future population momentum.

Figure 2 - Population Pyramids: Distribution of Population by Age and Sex (millions)

SOURCE: World Bank (1997a, 1997b) and projections using the model used there.

Continued substantial population growth can be expected in every developing-country region. Of the 1.8 billion additional people that can be expected in the next 25 years, East and Southeast Asia and the Pacific will contribute about 400 million (Figure 3). More will come from sub-Saharan Africa (almost 500 million) and South Asia (480 million). The increments will be smaller, but still substantial, in the Middle East crescent (260 million for this band of mostly Islamic countries stretching from Morocco to Kazakhstan) and in Latin America and the Caribbean (170 million). Although population growth will be most impressive in sub-Saharan Africa - leading to a population 75-percent larger by 2025 than it is today - other regions will also show impressive expansion. Latin America, for instance, will have 33 percent more people by 2025, and much of the increase will undoubtedly go to expand such already massive urban agglomerations as Mexico City.

Figure 3 - Population Increments by Decade in Developing-Country Regions (millions)

SOURCE: Estimated using the World Bank (1997a) projection model.